Pattern projection is commonly used in three-dimensional (“3D”) surface reconstruction for various measurements and inspections of electronic devices in the semiconductor industry. This approach and technique traditionally requires at least 3 images that are in different phases to compute the depth or height information. Conventional approaches may produce images with different phases by using micro-display technology such as digital micro-mirror devices (DMD) or liquid crystal on silicon (LCOS) devices. They may also produce images by physically moving the gratings, or by changing the relative positions between a projector and an object being inspected.
For instance, US Patent Publication number 2016/0014315A1 entitled “Apparatus and Method for Reconstructing a Three-Dimensional Profile of a Target Surface” describes an apparatus for reconstructing a 3D profile of a target surface of an object. The apparatus generates two images, one of which produces a pattern onto the target surface using a grating or reticle. One light path passes directly through or is reflected from the grating, and the other light path illuminates every part of the target surface as its light path does not include such a grating or reticle. With this design, a tilting of the grating can be controlled so that a focal plane of the illumination can be aligned with the object being inspected.
However, the system is only able to generate two sets of images. Moreover, assumptions regarding the fringe modulation or surface flatness are required in order to retrieve information relating to heights of points on the target surface that is measured. As there are instances where the fringe modulation on the target surface may change from point to point, such as where the object comprises different materials, the accuracy of the reconstruction will be adversely affected. On the other hand, if the measurable surface is constrained in relation to surface flatness, then the profiles of small height variations on the target surface cannot be reconstructed.
In another example, U.S. Pat. No. 7,079,666 entitled “System for Simultaneous Projections of Multiple Phase-Shifted Patterns for the Three-Dimensional Inspection of an Object” describes a 3D image grabber that allows for the simultaneous projection of multiple phase-shifted patterns onto an object, and the simultaneous acquisition of multiple images of these phase-shifted patterns. The image acquisition assembly is sensitive to different monochromatic lights for gathering incoming images illuminated by the plurality of phase-shifted patterns simultaneously. Different color patterns are thus simultaneously projected onto the object, and different colors are acquired as different images by the image acquisition assembly. Such phase-shifted pattern may be produced by a spectral splitter.
A shortcoming with generating different phase-shifted patterns having different lighting colors is that when an object being inspected has an uneven color spectrum distribution, the reflectivity intensities from the object would vary with the different lighting colors. In other words, the imaged intensities would be inconsistent across images that are illuminated with the different phases of light. This would inherently yield inaccurate height findings for objects with uneven color spectrum distributions.